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LT1011AIS8 Datasheet(PDF) 8 Page - Linear Technology |
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LT1011AIS8 Datasheet(HTML) 8 Page - Linear Technology |
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8 / 16 page  8 LT1011/LT1011A APPLICATIONS INFORMATION The input resistors should limit fault current to a reason- able value (0.1mA to 20mA). Power dissipation in the resistors must be considered for continuous faults, espe- cially when the LT1011 supplies are off. One final caution: lightly loaded supplies may be forced to higher voltages by large fault currents flowing through D1-D4. R3 and R4 limit input current to the LT1011 to less than 1mA when the input signals are held below V–. They may be eliminated if R1 and R2 are large enough to limit fault current to less than 1mA. Input Slew Rate Limitations The response time of a comparator is typically measured with a 100mV step and a 5mV to 10mV overdrive. Unfor- tunately, this does not simulate many real world situations where the step size is typically much larger and overdrive can be significantly less. In the case of the LT1011, step size is important because the slew rate of internal nodes will limit response time for input step sizes larger than 1V. At 5V step size, for instance, response time increases from 150ns to 360ns. See the curve “Response Time vs Input Step Size for more detail. If response time is critical and large input signals are expected, clamp diodes across the inputs are recom- mended. The slew rate limitation can also affect perfor- mance when differential input voltage is low, but both inputs must slew quickly. Maximum suggested common mode slew rate is 10V/ µs. Strobing The LT1011 can be strobed by pulling current out of the STROBE pin. The output transistor is forced to an “off” state, giving a “hi” output at the collector (Pin 7). Currents as low as 250 µA will cause strobing, but at low strobe currents, strobe delay will be 200ns to 300ns. If strobe current is increased to 3mA, strobe delay drops to about 60ns. The voltage at the STROBE pin is about 150mV below V+ at zero strobe current and about 2V below V+ for 3mA strobe current. Do not ground the STROBE pin. It must be current driven. Figure 4 shows a typical strobe circuit. Note that there is no bypass capacitor between Pins 5 and 6. This maximizes strobe speed, but leaves the compara- tor more sensitive to oscillation problems for slow, low – + LT1011 15V 5V RL 3k TTL OR CMOS DRIVE (5V SUPPLY) OUTPUT –15 8 6 1 7 4 1011 F04 Figure 4. Typical Strobe Circuit level inputs. A 1pF capacitor between the output and Pin 5 will greatly reduce oscillation problems without reducing strobe speed. DC hysteresis can also be added by placing a resistor from output to Pin 5. See step 5 under “Preventing Oscillation Problems.” The pin (6) used for strobing is also one of the offset adjust pins. Current flow into or out of Pin 6 must be kept very low (< 0.2 µA)whennotstrobingtopreventinputoffsetvoltage shifts. Output Transistor The LT1011 output transistor is truly floating in the sense that no current flows into or out of either the collector or emitter when the transistor is in the “off” state. The equivalent circuit is shown in Figure 5. R1 170 Ω R2 470 Ω Q1 V + D2 I1 0.5mA 1011 F05 V – D1 Q2 EMITTER (GND PIN) COLLECTOR (OUTPUT) OUTPUT TRANSISTOR Figure 5. Output Transistor Circuitry |
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